Method for longitudinal stretch of a parison to be blow molded

ABSTRACT

This injection blow molding method has special provision for maintaining the plastic of a parison on a core rod at the orientation temperature of the plastic; and stretching the plastic of the parison lengthwise of the core rod for orientation in the direction of one axis without any substantial increase in the diameter of the parison. After this orientation in one axis, the temperature is controlled to maintain an orientation temperature, and the plastic of the parison is then blown to a larger diameter so as to obtain orientation in another axis for &#34;bi-axial orientation&#34;. The core rod is covered by an elastic balloon, and provision is made for obtaining flow of the plastic on the core rod lengthwise of the rod during the first orientation operation.

BACKGROUND AND SUMMARY OF THE INVENTION

This is a division of application Ser. No. 441,811, filed Feb. 12, 1974now U.S. Pat. No. 3,905,738, issued Sept. 16, 1975.

When articles are made of plastic on blow molding machines, the plasticis substantially stronger if the plastic material has been stretched intwo directions at right angles to one another. This results in bi-axialorientation. With some plastics bi-axial orientation is necessary inorder to obtain a clear transparent article. With other materials thebi-axial orientation substantially increases the strength of the finalproduct even though no clear transparency can be obtained.

This invention provides an improved method for making bi-axiallyoriented products by blow molding. In order to obtain more flexibilityin handling the parison on the core rod, this invention uses a core rodwhich is covered with an elastic balloon. This balloon hugs the core rodwhen the balloon is deflated and the parison is applied over the surfaceof the balloon in the injection mold.

The core rod is transferred from the injection mold to a conditioningstation where the plastic of the parison is brought to orientationtemperature; that is, the temperature at which solidification begins.The parison is then stretched in the direction of its length byincreasing the length of the core rod and stretching the balloonlengthwise of the core rod with the plastic material coated over theballoon.

This stretching operation is performed in a mold with special provisionfor permitting movement of the plastic in the mold lengthwise of thecore rod and with no contact, or only light contact, between the plasticand the mold surfaces.

Following this initial stretching, the core rod and parison are moved toa second conditioning station and then to a blowing mold in which theparison is preferably blown to its final shape.

Other objects, features and advantages of the invention will appear orbe pointed out as the description proceeds.

BRIEF DESCRIPTION OF DRAWING

In the drawing, forming a part hereof, in which like referencecharacters indicate corresponding parts in all the views:

FIG. 1 is a diagrammatic top plan view of blow molding apparatus forobtaining bi-axial orientation in accordance with this invention;

FIG. 2 is a greatly enlarged sectional view through the mold in whichthe parison is stretched to provide orientation in one axis; and

FIG. 3 is a fragmentary, enlarged, sectional view taken on the line 3--3of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 shows a blow molding machine 10 which has five stations. Thefirst station is an injection station 12 in which plastic is injectedinto a mold cavity to apply a parison to core rods 14 which extend froman indexing head 16 that supports the core rods. The core rods 14 aresupported from the indexing head 16 in the usual manner.

After the injection step, the injection mold 12 opens and the indexinghead moves the core rods to a second station which is a conditioningstation 18 which is located at an angle of 72°, around the axis ofrotation of the indexing head, with respect to the injection station 12.

The apparatus shown in FIG. 1 being a five position machine, eachstation is located at an angle of 72° with respect to each adjacentstation and the indexing head 16 moves through an angle of 72° each timethe core rods 14 are to be shifted from one station to the next.

At the conditioning station 18, there are means for bringing the parisonto its orientation temperature by use of fluid from temperature controlapparatus 20. The construction of the conditioning station 18 and itstemperature control means 20 can be conventional and no furtherillustration of it is necessary for a complete understanding of thisinvention. The temperature to which molten plastic must be brought inorder to obtain orientation by stretching of the plastic is well knownand while it differs from one plastic to another, it is always thetemperature at which the molten material begins to solidify.

With the next angular movement of the indexing head 16, the core rodsare moved from the first conditioning station 18 to a secondconditioning station 24 which includes a mold 26 and temperature controlmeans 28. The construction and operation of this second conditioningstation is the principal concern of the present invention and it will bedescribed in more detail in connection with FIGS. 2 and 3. For thepresent is is sufficient to understand that the parison is stretchedlengthwise of the core rod, while maintained at orientation temperature,in the mold 26 at the second conditioning station 24.

The core rods are moved next to a blowing station 30 having a mold inwhich the parison is blown, preferably to its final contour, while itstemperature is maintained at approximately the orientation temperatureby temperature control means 32.

Beyond the blowing station, the core rods are transferred to a stripperstation 34 at which blown articles are stripped from the core rod in aconventional manner.

FIG. 2 shows a core rod 14 which has a shoulder 38 at one end forconnecting the core rod to the indexing head 16 by screws 40 whichthread into the face of the indexing head 16. These screws 14 also holda hub 42 securely connected to the indexing head 16. This hub 42 extendsaround the neck end of the core rod 14.

The core rod is hollow but not of uniform inside diameter. For a portionof the length of the core rod there is a tube 44 which is concentricwith the longitudinal axis of the interior of the core rod and this tube44 has an outside diameter substantially less than the inside diameterof the core rod along most of the length of the tube 44 within the corerod. There is provided, therefore, an annular chamber 46 between theoutside surface of the tube 44 and the inside surface of the core rodfor a substantial portion of the core rod length, including all of theright hand or neck portion of the core rod in FIG. 2. Near the righthand end of the tube 42 there is a shoulder 48 at which the interiordiameter of the core rod decreases to a bore 50 which has an insidediameter substantially equal to the outside diameter of the tube 44. Theend of the tube 44 extends into this bore 50 with a press fit or otherrigid connection of the tube 44 with the core rod 14.

Beyond the end of the tube 44 there is a chamber 52 within the core rod14 and this chamber 52 has passages 54 opening through its side wallsfor flow of fluid from the chamber 52 into an annular space 56 betweenthe outside surface of the core rod 14 and an inside surface of anelastic balloon 58 which covers the core rod. A plastic parison 60 coatsthe outside of the balloon 58.

When the core rod 14 is in the injection mold, the balloon 58 is fullydeflated and hugs the outside surface of the core rod 14. The parison 60is applied to the balloon 58 while the balloon is hugging the core rodand the initial expansion of the balloon to form the annular space 56 isobtained by a slight initial blowing of the balloon in the firstconditioning station 18.

The purpose of this initial blowing is to permit the circulation oftemperature controlling fluid along the inside surface of the balloon 58for temperature control of the parison 60 but this is the subject matterof another patent application and it is mentioned here merely for thepurpose of answering the question as to where the annular space 56 wasfirst produced.

The chamber 52 does not extend all the way to the end of the core rodbut the passages 54 are spaced along the length of the core rod for asubstantial distance beyond the chamber 52 and these additional passages54 are supplied with fluid from the chamber 52 through headers 62.

Temperature controlling fluid, for obtaining the necessary orientationtemperature of the parison 60, is circulated in contact with the insidesurface of the balloon 58. This circulation is obtained by passing thefluid, preferably liquid, through the tube 44 into the chamber 52 andfrom the chamber 52 through the passages 54 into the annular space 56 atvarious locations corresponding to the passages 54 which are spaced notonly longitudinally along the length of the core rod but also axiallyaround the circumference of the core rod. The fluid discharged into thespace 56 from the passages 54 flows lengthwise of the space 56, towardthe left in FIG. 2, and exhausts from the annular space 56 through inletpassages 64 leading from the annular space 56 through the wall of thecore rod and into the annular space 46 within which the fluid flowstoward the left and into an appropriate exhaust passage in the indexinghead 16.

The next portion 56 has its end bonded to the hub 42; and in theconstruction illustrated, the parison 60 is applied over a part of thehub 42 beyond the balloon 58.

At the free end of the core rod 14, that is, the end remote from theindexing head 16, there is a longitudinal bore 68 and there is a corerod extension element 70 which has a stem portion that slides in thebore 68 like a piston in a cylinder. This core rod extension element 70has a rounded head 72 and a threaded portion 74 adjacent to the roundedhead 72 with a nut 76 that screws over the threads 74 to clamp a washer78 against the portion of the balloon 58 that contacts with the underside of the rounded head 72. This construction is shown most clearly inFIG. 3 which is on a slightly larger scale than FIG. 2. The parison 60is omitted in FIG. 3 for clearer illustration.

The end of the balloon 58, remote from its neck portion, has an opening80 through which the core rod extension element 70 extends. Around theedges of the opening 80, the balloon has a lip 81 which fits into acomplementary recess in the back surface of the rounded head 72. The lip81 is clamped into this recess by the washer 78 when the nut 76 isscrewed up against the washer 78.

The pressure of the fluid in the annular space 56 tends to expand theballoon 58 and parison 60 to a larger diameter and also tends toincrease the length of the balloon since the core rod extension element70 can move longitudinally with respect to the otherwise fixed part ofthe core rod. The fluid pressure within the chamber 52, however, issubstantially higher than that in the annular space 56 because of thepressure drop through the passages 54. To provide greater force forstressing the balloon 58 and parison 60, there is a communicationpassage 84 between the chamber 52 and the left hand end of the bore 68.Pressure exerted through this passage 84, against the end of the corerod extension element 70 pushes the extension element 70 toward theright in the bore 68 with a cylinder-and-piston action to move the corerod extension element 70 toward the right in FIG. 2, as indicated by thedotted line position, and thereby stretch the parison 60.

This stretching of the balloon and parison takes place in a mold havingan upper section 86 and a lower section 88 which move toward and fromone another in accordance with conventional practice. When the moldparts 86 and 88 are in contact with one another; that is, the mold isclosed, they form a mold cavity 90 which is slightly larger in diameterthan the parison 60 when the parison is introduced into the mold cavity90. The cavity 90, however, is substantially longer than the parison 60,as will be evident from FIG. 2. The parison can expand only slightly inthe cavity 90 without coming in contact with the walls of the cavity;but the parison 60 can be stretched for a substantial distance, asindicated by the dot and dash lines in FIG. 2.

There is a plunger 92 which extends through an opening in the end of thecavity 90 and the end face of this plunger 92 is shaped to produce thedesired set or contour for the parison 60 at the end of the stretchingoperation. The balloon 58 tends to expand beyond the rounded head 72 asthe result of pressure within the balloon, but the rounded head 72restrains this expansion.

It is essential, however, to prevent the core rod extension element 70from moving all the way into contact with the end of the cavity 90; thatis, with contact with the end face of the plunger 92.

In order to limit the stroke of the core rod extension element 70, thereis a pin 94 extending through a fixed portion of the core rod 14 andthrough a slot 96, best shown in FIG. 2, in the stem or piston portionof the core rod extension element 70.

In addition to the control of the temperature of the parison 60 bycirculating temperature controlling fluid through the annular space 56within the balloon 58, the apparatus as shown in the drawing alsocirculates cooling fluid through the clearance between the outsidesurface of the parison 60 and the inside surface of the cavity 90. FIG.2 shows temperature control apparatus 28 diagrammatically and inposition to deliver the temperature controlling fluid through a passage98 in the upper mold section 86. There are exhaust passages 100 and 102at the other end of the cavity 90 for the flow of temperaturecontrolling fluid from the cavity 90 through passages indicateddiagrammatically by the reference character 104 and these passagespreferably lead to self-releasing regulators, that is, regulators thatpermit pressure to build up to a certain value and then reduce thepressure in accordance with the adjustment of the regulators.

The circulation of the pressure controlling fluid from the passage 98 tothe exhaust passages 100 and 102 also serve another purpose. Thepressure of this fluid in the clearance between the parison 60 and thesurface of the cavity 90 tends to limit the expansion of the parison 60in a radial direction. The pressure of this fluid is correlated with thepressure within the balloon 58 so that the plastic of the parison 60will not be expanded into firm contact with the surface of the moldcavity 90 until the parison has been stretched to the desired extent forproviding axial orientation in a longitudinal direction. It will beevident that if the parison 60 were pressed into firm contact with themold cavity 90 before the parison had been stretched, then friction ofthe parison against the surface of the cavity 90 would reduce the amountof stretching of the plastic near the outside surface of the parisonwhile that on the inside surface would stretch more easily and theorientation of the parison would not be uniform.

The plunger 92 slides in a bearing 100 carried by a fixed frame 112. Theplunger 92 is connected with a piston rod 114 which threads into theplunger and is held by a lock nut 116. This piston rod 114 extends froma cylinder 118 which is representative of means for moving the plunger92 in and out with respect to the mold cavity as desired. Other plungers92 can be substituted for that shown depending upon the desired contourof the end of the parison when it leaves the mold cavity 90 and theplunger 92 is also shaped so that it stretches the portion of theparison which extends across the end of the core rod and therebyprovides orientation of the end of the parison as well as its sidewalls. The cylinder 118 moves the plunger 92 as necessary to providestretch of the end plastic of the parison comparable to that provided bythe lengthening of the core rod.

The preferred embodiment of the invention has been illustrated anddescribed, but changes and modifications can be made and some featurescan be used in different combinations without departing from theinvention as defined in the claims.

What is claimed is:
 1. A method of orienting a parison on a core rod forsubsequent blow molding to form a hollow oriented article whichcomprises, conditioning the parison to its orientation temperature,introducing a temperature conditioning fluid between the core rod andthe inside of the parison for maintaining the parison at its orientationtemperature while stretching the parison lengthwise of the core rod, andmaintaining a fluid cushion on the outside of the parison to restrainany substantial circumferential stretching of the parison by said fluidinside the parison during said lengthwise stretching.
 2. The methoddescribed in claim 1 characterized by covering the core rod with aballoon, attaching the balloon to a neck end portion of the core rod,injection molding the parison onto the outside surface of the ballooncovering of the core rod while the balloon is deflated on and contactingthe core rod, and stretching the parison lengthwise of the core rod bystretching the balloon lengthwise without substantially increasing thetransverse cross section of the balloon.
 3. The method described inclaim 1 characterized by partially blowing the parison to separate itfrom the sides of the core rod while leaving the parison in contact witha movable portion of the core rod at an end of the core rod remote fromthe neck portion of the rod, and stretching the parison in saidpartially blown condition.
 4. The method described in claim 1characterized by distorting the plastic at the end of the parison whilestretching the parison lengthwise, the distortion being such as tochange the contour of the end of the parison to increase the totalradial extent of the end of the parison and thereby stretch the plasticmaterial radially while the lengthening of the core rod is stretchingthe sides of the parison longitudinally.
 5. The method of claim 1,further comprising passing fluid through the core rod to extend amovable end portion of the core so as to provide said lengthwisestretching.
 6. The method of claim 1, wherein said core rod is providedwith an elastic balloon covering the core rod with said balloon in thedeflated condition contacting the core rod, further comprising passingfluid through the core rod to extend a movable end portion of the coreso as to provide said lengthwise stretching, and without substantialcircumferential stretching of the balloon.
 7. The method of claim 1,further comprising circulating the temperature conditioning fluidthrough a first set of orifices in the core rod between the core rod andparison and out through the second set of orifices in the core rod, andmaintaining said fluid cushion on the outside of the parison to restrainany substantial circumferential stretching.
 8. The method of claim 1,wherein said core rod is provided with an elastic balloon covering thecore rod with said balloon in the deflated condition contacting the corerod, further comprising circulating the temperature conditioning fluidthrough a first set of orifices in the core rod between the core rod andballoon and out through a second set of orifices in the core rod, andmaintaining said fluid cushion on the outside of the parison to restrainany substantial circumferential stretching.